1. Field of The Invention
The present invention relates to a submarine optical cable comprising a polymeric composition suitable for filling the interstices within said cable, in order to control a longitudinal flow of water accidentally penetrated inside said cable.
2. Background Art
Submarine optical cables are subjected, in case of accidental rupture thereof, to a sudden ingress of a sea-water flow at high pressure (e.g. 100 bar, when the cable is at 1000 m below the sea level). Such high pressure water flow may propagate for a relevant length inside the cable if suitable water blocking means are not provided in the cable, thus damaging a remarkable portion of said cable which has then to be replaced.
A number of cables designed for submarine installation are known in the art.
For instance, U.S. Pat. No. 4,684,213 relates to a submarine cable comprising a pressure resistant steel tube containing optical fibers, surrounded by two layers of steel wires and by an outer metal tube made of copper or aluminum. Dams of a sticky compound and/or of a jelly of plastic material are disposed at regular intervals inside the central tube and in the gaps between the lay of wires disposed between the central tube and the outer tube.
U.S. Pat. No. 5,125,062 discloses an undersea cable comprising a central metallic tube, filled with a sealing compound, e.g. silica gel, and containing optical fibers embedded therein, said tube being surrounded by a helical lay of metallic (preferably steel) wires. Interstices between wires and between the helical lay and the central tube are filled with a sealing material, such as a polyurethane resin, which opposes longitudinal propagation of water along the cable. Alternatively, the central tube can be made of plastic and in this case the helical lay also presents the characteristics of an arch for withstanding pressure.
U.S. Pat. No. 4,726,649 relates to a submarine cable wherein the voids inside the cable are filled with a material having adequate elongation property and creep characteristics, in order to withstand the high pressure water flow following an accidental rupture of the cable. As filling material, a polyurethane resin is disclosed, said resin being comprised of not less than 10% by weight, preferably not less than 30% by weight of a hydrocarbon polyol/polyisocyanate mixture and from about 5 to 90%, preferably from 10% to 70% of a hydrocarbon oil. In the working examples a paraffinic oil is employed as a mineral oil, in a maximum amount of 56% by weight.
The applicant has now observed that while prior art resin filling compositions, in particular polyurethane based resins, has been developed with selected mechanical characteristics allowing the resin to effectively block the longitudinal water flow inside a submarine cable, (e.g. following an accidental rupture of the same), no attention has been paid in the prior art cables to the mechanical interaction between the filling compositions and the optical fibers disposed inside the optical core.
The Applicant has in fact observed that commercial polyurethane-based filling composition, when cross-linked, reach a relative high hardness. The Applicant has further observed that mechanical stresses can be generated in the cable structure, in particular during the manufacturing process of the cable, such as during the stranding of metal armoring wires around the optical core (e.g. as a consequence of a slight variations in the circular cross-section of the stranded metallic wires). Due to the relatively high hardness of the resin filling material disposed around the optical core, said mechanical stresses can be transmitted onto the optical core, thus causing a permanent deformation on the structure of the same, with consequent possible attenuation of the signal transmitted by the optical fiber contained therein.
Applicant has noticed that this problem becomes much more relevant when Large Effective Area (LEA) fibers are used in the optical cable, which fibers are much more sensitive to bending-induced losses (also known as the microbending and macrobending phenomena) than standard dispersion-shifted fibers (SDS fibers). The term LEA fibers is intended to encompass those optical fibers having a large effective area, in particular optical fibers having an effective area of at least 7 μm2 or greater. In particular, said LEA fibers may be used in wavelength-division-multiplexing (WDM) and high bandwidth systems.
The Applicant has now observed that the resin filling material, while being capable of guaranteeing the desired water-blocking performances, shall have at the same time a relatively reduced hardness, in order not to negatively affect the signal transmission within the optical fiber in case of mechanical stresses produced onto the cable, and particularly onto the optical core.
The Applicant has further observed that certain components of conventional polyurethane resins may negatively interact with some material forming the structural elements of the cable. In particular, ester compounds which may be used as plasticizers in conventional polyurethane resin composition may negatively interact with the polymeric material of the optical core of submarine cables. The Applicant has thus found that by suitably formulating a polyurethane resin composition, in particular by using a relatively high amount of mineral oil and a rather limited amount of ester compounds, it is possible to obtain a good compatibility between the polyurethane resin and other polymeric materials of the cable.